Unlike other tissues which can survive extended periods of hypoxia, brain tissue is particularly sensitive to deprivation of oxygen or energy. Permanent damage to neurons can occur during brief periods of hypoxia, anoxia or ischemia. Neurotoxic injury is known to be caused or accelerated by certain excitatory amino acids (EAA) found naturally in the central nervous system (CNS). Glutamate (Glu) is an endogenous amino acid which has been characterized as a fast excitatory transmitter in the mammalian brain. Glutamate is also known as a powerful neurotoxin capable of killing CNS neurons under certain pathological conditions which accompany stroke and cardiac arrest. Normal glutamate concentrations are maintained within brain tissue by energy-consuming transport systems. Under low energy conditions which occur during conditions of hypoglycemia, hypoxia or ischemia, cells can release glutamate. Under such low energy conditions the cell is not able to take glutamate back into the cell. Initial glutamate release stimulates further release of glutamate which results in an extracellular glutamate accumulation and a cascade of neurotoxic injury.
It has been shown that the sensitivity of central neurons to hypoxia and ischemia can be reduced by either blockage of synaptic transmission or by the specific antagonism of postsynaptic glutamate receptors [see S. M. Rothman et al, Annals of Neurology, Vol. 19, No. 2 (1986)]. Glutamate is characterized as a broad spectrum agonist having activity at three neuronal excitatory amino acid receptor sites. These receptor sites are named after the amino acids which selectively excite them, namely: Kainate (KA), N-methyl-D-aspartate (NMDA or NMA) and quisqualate (QUIS). Glutamate is believed to be a mixed agonist capable of binding to and exciting all three receptor types.
Neurons which have EAA receptors on their dendritic or somal surfaces undergo acute excitotoxic degeneration when these receptors are excessively activated by glutamate. Thus, agents which selectively block or antagonize the action of glutamate at the EAA synaptic receptors of central neurons can prevent neurotoxic injury associated with anoxia, hypoxia or ischemia caused by stroke, cardiac arrest or perinatal asphyxia.
Aminophosphonic acids have been investigated as neurotransmitter blockers [see M. N. Perkins et al, Neuroscience Lett., 23, 333 (1981); and J. Davies et al, Neuroscience Lett., 21, 77 (1981)]. In particular, compounds such as 2-amino-4-(2-phosphonomethylphenyl)-butyric acid and 2-(2-amino-2-carboxy)ethylphenylphosphonic acid have been synthesized for evaluation as antagonists in blocking the action of the neurotransmitter compounds L-glutamic acid and L-aspartic acid [K. Matoba et al, "Structural Modification of Bioactive Compounds II. Syntheses of Aminophosphonic Acids", Chem. Pharm. Bull., 32, (10) 3918-3925 (1984)].
U.S. Pat. No. 4,657,899 to Rzeszotarski et al described a class of .omega.-[2-(phosphonoalkylenyl)phenyl]-2-aminoalkanoic acids characterized as being selective excitatory amino acid neurotransmitter receptor blockers. These compounds are mentioned for use as anticonvulsants, antiepileptics, analgesics and cognition enhancers. Typical compounds of the class include 3-[2-phosphonomethylphenyl]-2-aminopropanoic acid and 3-[2-(2-phosphonoethyl)phenyl]-2-aminopropanoic acid.
An analogue of 2-amino-7-phosphonaheptanoic acid, namely 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid [CPP], has been reported as a potent and selective NMDA antagonist in an evaluation of CPP binding to rat brain hippocamal tissue [D. E. Murphy et al, J. Pharmacol. Exp. Ther., 240 (3), 778-784 (1987)].
Several classes of imidazopyridine compounds are known having various pharmaceutical uses. For example, EP# 120,589 published 3 Oct. 1984 describes certain imidazo-(1,2-a)pyridinylheterocyclic compounds for use as cardiotonic and antiulcer agents. Schering EP# 33,094 published 5 Aug. 1981 describes 3,8-disubstituted-imidazo-(1,2-a)pyridine compounds for use as antisecretory and cyto-protective agents. Synthelabo U.S. Pat. No. 4,650,796 published 19 Feb. 1986 describes 2-phenyl-3-acylaminomethylimidazo pyridine compounds as anxiolytic, hypnotic and anticonvulsant agents. Synthelabo U.S. Pat. No. 4,501,745 describes imidazo-(1,2-a)-pyridinealkanoic acid derivatives as anxiolytic, hynotic and anticonvulsant agents. Schering U.S. Pat. No. 4,450,164 describes phosphonic acid derivatives of imidazo(1,2-a) pyridine compounds for use as treatment of gastrointestinal diseases such as ulcers.
Ciba-Geigy U.S. Pat. No. 4,746,653 describes phosphono-substituted 2-carboxy-2,3-dihydro or perhydroindolyl derivatives for use as NMDA antagonists to treat cerebral ischemia.